Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features

By M. R. McGovern, J. D. Kirkpatrick, I. McLean, A. Burgasser, L. Prato, P. Lowrance. 2003

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We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project to study gravity sensitive features as indicators of youth in brown dwarfs. Low-resolution (R ~ 2000) J-band and optical (R ~ 1000) observations using NIRSPEC and LRIS at the W. M. Keck Observatory reveal transitions of TiO, VO, K I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type giants and in old field dwarfs, we show that they are sensitive to the gravity (g = GM/R2) of the object. Using low-gravity spectral signatures as age indicators, we observed and analyzed J-band and optical spectra of two young brown dwarfs, G196-3B (20-300 Myr) and KPNO Tau 4 (1-2 Myr) and two possible low-mass brown dwarfs in the σ Orionis cluster (3-7 Myr). We report the identification of the ϕ bands of TiO near 1.24 μm and the A-X band of VO near 1.18 μm together with extremely weak J-band lines of K I in KPNO Tau 4. This is the first detection of TiO and VO in the J band in a substellar-mass object. The optical spectrum of KPNO Tau 4 exhibits weak K I and Na I lines, weak absorption by CaH, and strong VO bands, also signatures of a lower gravity atmosphere. G196-3B shows absorption features in both wavelength regions, like those of KPNO Tau 4, suggesting that its age and mass are at the lower end of published estimates. Whereas σ Ori 51 appears to be consistent with a young substellar object, σ Ori 47 shows signatures of high gravity most closely resembling an old L1.5/L0 and cannot be a member of the σ Orionis cluster.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c73e35920ac6b1dabf4167e8930d0afb4102af59

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THE BROWN DWARF KINEMATICS PROJECT I. PROPER MOTIONS AND TANGENTIAL VELOCITIES FOR A LARGE SAMPLE OF LATE-TYPE M, L, AND T DWARFS (2008, background): “Currently defined subgroups of late-type M, L and T dwarfs include 1) low surface-gravity, very low-mass objects (e.g. McGovern et al. 2004, Kirkpatrick et al. 2006, Allers et al. 2007, Cruz et al. 2007), 2) old, metal-poor ultracool subdwarfs (e.g. Burgasser et al. 2003a, Lépine et al. 2003,…”
A Spitzer Census of the IC 348 Nebula (2007, background): “Because near-IR H2O absorption bands are stronger in young objects than in field dwarfs at a given optical spectral type (Luhman & Rieke 1999; Lucas et al. 2001; McGovern et al. 2004), spectral types of young objects derived from H2O with dwarf standards will be systematically too late.”
An Infrared Spectroscopic Sequence of M, L, and T Dwarfs (2004, unknown): “…L8 2002 Dec 09 212 252 HD 28813 SDSS J1254 0122 ...................... T2 2001 May 06 212 1800 HD 109309 2MASS J0559 1404................... T5 2002 Dec 09 212 2590 HD 47596 7 The band of TiO has been detected in the spectrum of the young brown dwarf KPNO Tau 4 by McGovern et al. (2004).”
The Spectroscopically Determined Substellar Mass Function of the Orion Nebula Cluster (2004, background): “This figure illustrates the dramatic decrease in strength of the K I lines with decreasing surface gravity (see also McGovern et al. 2004 and Gorlova et al. 2003).”
The Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs (2012, background): “The variation of gravity-sensitive lines with age has been examined in detail in several studies (Kirkpatrick et al. 2006, McGovern et al. 2004, Rice et al. 2010), which have shown that differences of ∼ 1 dex in log(age) can be detected within a sample of objects (Kirkpatrick et al. 2008).”
ExoMol line lists – XVIII. The high-temperature spectrum of VO (2016, background): “VO is one of the dominant species in the spectra of young hot brown dwarfs (McGovern et al. 2004; Kirkpatrick et al. 2006; Peterson et al. 2008).”
Meeting the Cool Neighbors. IX. The Luminosity Function of M7-L8 Ultracool Dwarfs in the Field (2006, methodology): “In particular, we have used enhanced VO bands (7330–7530, 7850–8000 Å), less-broad K I doublet (7665, 7699 Å), and weaker Na I doublet (8183, 8195 Å) as diagnostics (Mart́ın et al. 1996; Gizis et al. 1999; Gorlova et al. 2003; McGovern et al. 2004; Kirkpatrick et al. 2006a).”
THE HAWAII INFRARED PARALLAX PROGRAM. II. YOUNG ULTRACOOL FIELD DWARFS (2016, background): “…the expected ∼300 Myr timescale dis- appearance of low-gravity features (e.g. Kirkpatrick et al. 2008; Allers & Liu 2013a; Smith et al. 2015), with ample evidence that objects at ∼100 Myr still have distinct gravity signatures (e.g. Mart́ın et al. 1999b; McGovern et al. 2004; Aller et al. 2016).”
BANYAN. VII. A NEW POPULATION OF YOUNG SUBSTELLAR CANDIDATE MEMBERS OF NEARBY MOVING GROUPS FROM THE BASS SURVEY (2015, background): “…atmospheres, causing deeper absorption bands at 7300-7550 and 7850–8000 Å in the optical and 1.06µm in the NIR (These effects are discussed in more detail by Gorlova et al. 2003; McGovern et al. 2004; Kirkpatrick et al. 2006, 2008; Cruz et al. 2009; Allers & Liu 2013 and Canty et al. 2013).”
New spectral types L and T (2005, unknown): “McGovern et al. (2004) argue that the L1.”
Characterizing Young Brown Dwarfs Using Low-Resolution Near-Infrared Spectra (2006, background): “Older field dwarfs have intrinsically bluer near-IR spectra and shallower H2O absorption features than do younger brown dwarfs, effects noted byKirkpatrick et al. (2006) and McGovern et al. (2004).”
BANYAN. II. VERY LOW MASS AND SUBSTELLAR CANDIDATE MEMBERS TO NEARBY, YOUNG KINEMATIC GROUPS WITH PREVIOUSLY KNOWN SIGNS OF YOUTH (2013, background): “Metchev et al. (2008) report an age estimate of 60 to 300 Myr for 2MASS J10042066+5022596, however McGovern et al. (2004) state that it could be younger.”
A NEAR-INFRARED SPECTROSCOPIC STUDY OF YOUNG FIELD ULTRACOOL DWARFS (2013, background): “Not surprisingly, many of these same spectral features are seen in the spectra of young, late-M and early-L type brown dwarfs in star-forming regions (e.g. Lucas et al. 2001; Gorlova et al. 2003; McGovern et al. 2004; Allers et al. 2007; Lodieu et al. 2008).”
Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity (2004, background): “Recent investigations of spectroscopic gravity indicators for L and T dwarfs include those by Lucas et al. (2001), Burgasser et al. (2003b), Gorlova et al. (2003), Martı́n & Zapatero Osorio (2003), and McGovern et al. (2004).”
Discovery of a Very Young Field L Dwarf, 2MASS J01415823–4633574 (2005, background): “These pairs of KI lines are known to be sensitive gravity indicators (Gorlova et al. 2003, McGovern et al. 2004) and together yield a best fit of log(g) = 4.0±0.5.”
Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich? (2008, background): “Other gravity diagnostics such as atomic K I line strengths and FeH absorption (McGovern et al. 2004) may support this hypothesis.”
Discovery of Two T Dwarf Companions with the Spitzer Space Telescope (2006, background): “In comparison, these lines become weaker with lower gravity for late-M and early-L dwarfs (McGovern et al. 2004; Kirkpatrick et al. 2006) but grow stronger with lower gravity at late-T types (Knapp et al. 2004).”
YOUNG L DWARFS IDENTIFIED IN THE FIELD: A PRELIMINARY LOW-GRAVITY, OPTICAL SPECTRAL SEQUENCE FROM L0 TO L5 (2008, background): “…of VO and FeH absorption bands at low gravities has been established empirically: In both late-M giants and young dwarfs, the VO bands at 7300–7550 Å and 7850–8000 Å are strong while the FeH band at 1.2 µm is weak (Mart́ın et al. 1996; Gizis 2002; Briceño et al. 2002; McGovern et al. 2004).”